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Plain CSS Nesting: Draft 1

(Issue, Changelog)

Table of Contents

Background

This section is non-normative.

Browsers have recently begun implementing the CSS Nesting module, which adds native support for Sass-like nesting. While we can't yet support this directly in Sass without causing a colossal set of breaking changes (see the blog for details), we can support it in plain CSS files (distinguished with the .css extension).

Summary

This section is non-normative.

This proposal adds support for parsing nested rules and the parent selector & in plain CSS contexts. This nesting is not resolved in any way; it's passed through to the output as-is.

Design Decisions

The &foo Syntax

This spec does not include support for the &foo syntax in plain CSS nesting. The future of this syntax is open for debate and it can be adequately represented as foo&, so Sass won't support it for now.

Procedures

Parsing Text as CSS

This modifies the existing procedure for parsing text as CSS.

Adjust the list of productions that should produce errors as follows:

  • Remove "A style rule appearing within another style rule".

  • Replace "The parent selector &, either in a selector or a declaration value" with "The parent selector & in a declaration value".

  • Add "A style rule whose selector contains a trailing combinator."

    While the bogus combinators deprecation is in place, style rules with trailing combinators that don't have nested rules will produce warnings. Those with nested rules will produce errors since Sass never parsed them successfully in the first place.

Add the following to the list of parsing differences:

  • A ParentSelector may appear anywhere in a CompoundSelector, rather than just as the first SimpleSelector.

  • A ParentSelector may not have a suffix.

Semantics

Style Rules

Replace the existing semantics for style rules with:

Differences are highlighted in bold.

To execute a style rule rule:

  • Let selector be the result of evaluating all interpolation in rule's selector and parsing the result as a selector list.

  • If rule's stylesheet wasn't parsed as CSS:

    Checking whether rule's stylesheet is CSS ensures that the plain CSS behavior occurs even when plain CSS is evaluated in a Sass context, such as through a nested @import or a meta.load-css() call.

    • If there is a current style rule:

      • If selector contains one or more parent selectors, replace them with the current style rule's selector and set selector to the result.

      • Otherwise, nest selector within the current style rule's selector using the descendant combinator and set selector to the result.

    • Otherwise, if selector contains one or more parent selectors, throw an error.

  • Let css be a CSS style rule with selector selector.

  • Execute each child child of rule.

  • If css contains any children and selector is bogus, throw an error.

  • Remove any complex selectors containing a placeholder selector that begins with - or _ from css's selector.

  • Unless css's selector is now empty:

    • If rule's stylesheet was parsed as CSS and there is a current style rule or a current at-rule, append css to whichever of the two exists, or the innermost if both exist.

    • If there is a current at-rule, append css to its children.

      This was intended to be in the current spec, but was overlooked.

    • Otherwise, append css to the current module's CSS.

Serialization

Parent Selector

To serialize a parent selector, emit the character &.

A parent selector can only appear in a serialized selector if it was parsed from plain CSS, which doesn't allow it to have a suffix.